Inclusion Criteria:
* Patient must have radiographically confirmed, newly diagnosed or recurrent pediatric-type high grade glioma: Diffuse Midline Glioma, H3 K27-altered; Diffuse hemispheric glioma, H3 G34-mutant, Diffuse pediatric-type HGG, grade III and IV H3-wildtype and IDH-wildtype with imaging and/or pathology consistent with a DMG, (including spinal cord tumors); or Recurrent malignant tumors involving the brainstem or posterior fossa (choroid plexus carcinoma, CNS embryonal tumors, and pineoblastoma).
* Karnofsky ≥ 50 for patients \> 16 years of age or Lansky ≥ 50 for patients ≤ 16 years of age. Participants who are unable to walk because of paralysis, but who are up in a wheelchair, will be considered ambulatory for the purpose of assessing the performance score.
* If clinically indicated, participants must be willing to provide adequate tissue for PK analysis, either from a surgical biopsy or needle biopsy.
* Aged ≥5 to ≤18 years.
* Patients recurrent or progressive disease must have recovered from all acute side effects of prior therapy.
* From the projected start of scheduled study treatment, the following time periods must have elapsed: At least 7 days after last dose of a biologic agent or beyond time during which adverse events are known to occur for a biologic agent, 5 half-lives from any investigational agent, 4 weeks from cytotoxic therapy (except 23 days for temozolomide and 6 weeks from nitrosoureas), 6 weeks from antibodies, or 4 weeks (or 5 half-lives, whichever is shorter) from other anti-tumor therapies.
* Prior use of temozolomide during radiation at maximum of the standard pediatric dosing (defined as 90 mg/m2 /dose continuously during radiation therapy for 42 days) or dexamethasone is allowed.
* Corticosteroids: Patients who are receiving dexamethasone must be on a stable or decreasing dose for at least 3 days prior to baseline magnetic resonance imaging (MRI) scan.
* Surgical Resection: Patients may not have had a surgical resection within four weeks of starting NEO100 and any post-surgical complications must have resolved prior to initiation of NEO100.
* Radiation Therapy: Patients may not receive radiation therapy within four weeks of starting NEO100. Patients may not be enrolled if they require radiation therapy during Cycle 1 (the DLT Period). Patients may have SOC radiation therapy in Cycle 2 or beyond but will require a second DLT evaluation period to participate.
* Peripheral absolute neutrophil count (ANC) ≥ 1000/mm\^3 (1.0g/l) AND
* Platelet count ≥ 100,000/mm\^3 (100x10\^9/l) (transfusion independent, defined as not receiving platelet transfusions for at least 7 days prior to enrollment).
* Creatinine clearance or radioisotope glomerular filtration rate (GFR) ≥ 70 mL/min/1.73 m\^2.
* Bilirubin (sum of conjugated + unconjugated) ≤ 1.5 x upper limit of normal (ULN) for age.
* Serum glutamate pyruvate transaminase (SGPT) (alanine aminotransferase (ALT)) ≤ 2 x ULN.
* Serum albumin ≥ 2 g/dL.
* No or mild renal impairment (i.e., 60 mL/min or 1.73m2 with Schwartz equation).
* No evidence of dyspnea at rest, no exercise intolerance due to pulmonary insufficiency, and a pulse oximetry of \> 92% while breathing room air.
* Participants with seizure disorder may be enrolled if seizure disorder is well controlled.
* The effects of the study drugs on the developing human fetus are unknown. For this reason, females of child-bearing potential and males must agree to use adequate contraception. Adequate methods include: hormonal or barrier method of birth control; or abstinence prior to study entry and for the duration of study participation. Should a woman become pregnant or suspect she is pregnant while she or her partner is participating in this study, she should inform her treating physician immediately. Males treated or enrolled on this protocol must also agree to use adequate contraception prior to the study and for the duration of study participation.
* A legal parent/guardian and patient must be able to understand, and willing to sign, a written informed consent (parental permission) or assent document, as appropriate.
Exclusion Criteria:
* Diagnosis of any pediatric-type glioma not described in inclusion criteria.
* Participants who are currently receiving another investigational drug. Investigational imaging agents or agents used to enhance tumor visibility on imaging or during tumor biopsy/resection should be discussed with the Medical Monitors.
* Participants who are currently receiving other anti-cancer agents.
* Participants with a known disorder that affects their immune system, such as human immunodeficiency virus (HIV) or hepatitis B or C, or an auto-immune disorder requiring systemic cytotoxic or immunosuppressive therapy. Note: Participants that are currently using inhaled, intranasal, ocular, topical or other non-oral or non-intravenous (IV) steroids are not necessarily excluded from the study but need to be discussed with the Medical Monitor.
* A marked baseline prolongation of QT/QTc interval (e.g., repeated demonstration of a QTc interval \>450 milliseconds (ms).
* A history of additional risk factors for TdP (e.g., heart failure, hypokalemia, family history of Long QT Syndrome), or the use of concomitant medications that prolong the QT/QTc interval.
* Participants with uncontrolled infection or other uncontrolled systemic illness.
* Female patients of childbearing potential must not be pregnant or breast-feeding. Female patients of childbearing potential must have a negative serum or urine pregnancy test prior to the start of therapy (as clinically indicated).
* Active illicit drug use or diagnosis of alcoholism.
* History of allergic reactions attributed to compounds of similar chemical or biologic composition as the agents used in study.
* Evidence of disseminated disease, including diffuse leptomeningeal disease or evidence of CSF dissemination as determined based upon available clinical details and not a study required MRI study or CSF test.
* Known additional malignancy that is progressing or requires active treatment within 3 years of start of study drug.
* Concomitant use of potent CYP3A4/5 inhibitors during the treatment phase of the study and within 72 hours prior to starting study drug administration.
* Concomitant use of potent CYP3A4/5 inducers, which include enzyme inducing antiepileptic drugs (EIAEDs), during the treatment phase of the study and within 2 weeks prior to starting treatment. Concurrent corticosteroids are allowed.
